1. Field of the Invention
The present invntion relates to a movement detector for use in television signal processing.
2. Related Art
There are a number of applications in television signal processing which benefit from motion-adaptive processing. For example it is desirable to operate noise reduction methods in stationary parts of a scene but not in moving parts. In the field of digital television data rate reduction there are often different algorithms suitable for moving and for stationary picture areas. Also in line-rate conversion, different interpolation schemes are sometimes needed depending on scene motion. In each of these cases a number of difficulties arise in defining suitable movement detection processing. For example noise (or subcarrier in an environment where composite colour coding is used) can masquerade as scene movement. There is one area of difficulty however which arises specifically in the application of line-rate conversion. This will now be described for the specific application of up-conversion from a line-format with 2:1 interlace to a non-interlaced format, although conversion to other line formats will show one some difficulty.
In order to achieve improvement in picture quality it is possible to "upconvert" an interlaced signal for display with a higher line-rate.
This may take place, for example, in a satellite broadcasting receiver designed for displaying extended definition signals. The relationship between an incoming interlaced line-format and a resulting non-interlaced line format, for example, is illustrated in FIG. 1. Crosses indicate received interlace lines and circles indicate the additional lines which need to be interpolated to provide a non-interlaced scan. The dot is taken as an example line (in "field 0") to be interpolated and the "surrounding" (in space and time) received interlace lines are labelled A,B,C and D. If the appropriate area of the scene is stationary, then the best method of interpolation for the new line is to base it on values taken from the same vertical position. For example an average of A and B would be suitable (a form of "temporal interpolation"). This would enable the full vertical resolution capabilities of the received interlaced lines to be achieved. If there is some scene movement in the vicinity of lines A,B,C and D however then values from positions A and B would provide an erroneous interpolation and a better option would be to use lines from the same field. For example an average of C and D (a form of "vertical interpolation") could be suitable. If however C and D were used for interpolation in a stationary scene, then vertical resolution would be sacrificed and much of the picture quality improvement promised by this technique would be lost. A movement detection circuit is therefore required in order to provide a control signal for determining the appropriate interpolation procedure.
One approach would be to use an absolute frame difference signal i.e. k=1A-B1 where k provides the basis for this control signal. In general, movement will give rise to a significant frame difference value while stationary picture detail will yield a zero frame-difference value. Thus a large frame difference signal is required and a very small frame difference signal could be used to indicate that temporal interpolation is required. Usually there will be more than two options for interpolation so that the transition in a scene from one type to the next would not be so abrupt. Typically there could be a number of options ranging from purely vertical interpolation at one extreme to purely temporal interpolation at the other. Intermediate interpolators would use lines displaced both vertically and temporally.
There is a fundamental difficulty which arises with such movement detection. This is because certain types of movement "bypass" the detector and result in a zero frame-difference signal. This difficulty does not arise in the systems based on a single scanning standard, such as noise reducers, and is thus specific to the application of line-rate conversion. For example consider a horizontally moving bar as illustrated in FIG. 2. As in FIG. 1, field 0 represents the current field in which a line is to be interpolated while field-1 is the previous field and field 1 is the subsequent field. The frame difference signal which results from such a bar is illustrated for field 0 in FIG. 3a. FIG. 3b shows the corresponding position of the bar itself. It can be seen that during the bar itself the framedifference value is zero even though the bar is moving. This would give rise to an inappropriate type of interpolation being used for lines within the bar. This effect can occur in practice and it can give objectionable results, especially with critical picture material such as electronically operated moving captions. This type of difficulty is not peculiar to a frame-difference approach to movement detection. Any movement-detection algorithm which acts on a local basis and which takes lines from only fields -1, 0 and 1 would encounter a similar difficulty. Indeed the desired movement information is simply not available in the received interlaced signal. Theory shows that a high vertical frequency which is stationary and this type of movement are indistinguishable on a local basis. It can be seen however that there is a frame-difference signal in FIG. 3a to the left and to the right of the bar position. Therefore if some "spreading" of the frame-difference signal is applied it is possible to alleviate this difficulty. This is the approach which has been adopted so far by research workers in this field. Typically a low-pass filter is used to "spread" the frame-difference signal. There is however a difficult compromise to reach concerning the degree of spreading to be applied. Too much would unduly restrict the resolution of the movement detector. Too little, on the other hand, would too severely limit the speed of movement range over which the technique would be effective. This is because the spacing between the frame difference pulses illustrated in FIG. 3B is directly proportional to the speed of movement of the bar. In addition a horizontal spreading technique is not effective for the similar difficulties which can occur with vertical motion.